1. Technical Field of the Invention
This invention relates generally to wireless communications and more particularly to radio receivers used within such wireless communication systems.
2. Description of Related Art
Wireless communication systems are known to include a plurality of wireless communication devices that communicate over wireless communication channels, which are supported by wireless communication infrastructure equipment (e.g., base stations, access points, system controllers, wide area network interfaces, local area network interfaces, et cetera). Each wireless communication device, which may be a radio, cellular telephone, station coupled to a personal digital assistant, personal computer, laptop, et cetera, includes a radio transmitter and a radio receiver. The radio transmitter includes a baseband processor, one or more intermediate frequency stages, filters, and a power amplifier coupled to an antenna. The baseband processor encodes and/or modulates, in accordance with a wireless communication standard such as IEEE 802.11a, IEEE802.11b, Bluetooth, Global System for Mobile communications (GSM), Advanced Mobile Phone Service (AMPS), et cetera, to produce baseband signals. The one or more intermediate frequency stages mix the baseband signals with one or more local oscillations to produce a radio frequency signal. The filter filters the radio frequency signal to remove unwanted frequency components and the power amplifier amplifies the filtered radio frequency signal prior to transmission via the antenna.
A radio receiver is known to include a low noise amplifier, one or more intermediate frequency stages, filters, analog to digital converters, and a receiver baseband processor. The low noise amplifier amplifies radio frequency (RF) signals received via an antenna and provides the amplified RF signals to the one or more intermediate frequency stages. The one or more intermediate frequency stages mixes the amplified RF signal with one or more local oscillations to produce a receive baseband signal. The analog to digital converters convert the received baseband signal into a digital baseband signals. The receiver baseband processor, in accordance with a particular wireless communication standard, decodes and/or demodulates the digital baseband signals to recapture data therefrom.
As is further known, the magnitude of an RF signal received by the receiver may vary by 80 dB or more. As such, the receiver includes an automatic gain control module to control the gain of the low noise amplifier, the intermediate frequency stage or stages, the filters and/or the analog-to-digital converters to keep the magnitude of the received signal at levels within the dynamic operating range of the circuitry comprising the receiver.
Prior to receiving a valid RF signal, the automatic gain control sets the gain of the receiver at a level to detect the presence of the weakest signal allowed by the governing standard. As such, the gain of the receiver, which includes multiple gain stages, is set very high. With the gain of the receiver set in this manner, the receiver measures the signal strength of what it is receiving and compares it to a threshold. In the absence of a valid signal, the receiver is receiving noise. To distinguish between noise and a weak valid signal, the automatic gain control sets the gain of the receiver such that the signal strength of the noise is a fixed power level below a potentially valid signal threshold (e.g., 10 dB below the threshold).
An issue with this signal detection approach is that, over time, the signal strength of the noise varies due to environmental changes in which the receiver is located. If these variations are not accounted for, valid signals may be missed (i.e., the sensitivity of the receiver is reduced) or the receiver will produce false/positive signal detections (i.e., the signal level of the noise will approach the threshold). The former results when the signal strength of the noise decreases and the later occurs when the signal strength of the noise increases.
To reduce the false/positive signal detects and the decreased receiver sensitivity, many receivers have the automatic gain control adjust the gain stages of the receiver to keep the signal strength of the noise at the predetermined value below the threshold. An issue with this approach is that the gain increments/decrements are finite and that to achieve the desired adjusted gain of the multiple gain stages; several of the gain stages are adjusted. If the gain stages are adjusted at the same time, a significant fluctuation in the signal level may occur, causing a false positive signal detect. To avoid this, the gain stages are adjusted sequentially, which requires a fair amount of time. Thus, if a valid but weak signal is received while the gain is being adjusted, the signal may be missed, especially for signals having a short training sequence such as those prescribed in the IEEE 802.11 standard.
Therefore, a need exists for a method and apparatus for quickly detecting the presence of a valid signal and improving receiver sensitivity in comparison to the above.